Here's what our constitution says about it (Google Translated):
"The Coat of Arms of Spain must appear on:
The flags that fly abroad or are displayed inside the headquarters of the constitutional organs of the State; the buildings and establishments of the central, institutional, regional, provincial or insular and municipal Administration of the State; military public buildings and barracks, ships, aircraft and any other establishments of the Armed Forces and State Security Forces, as well as units of both Forces entitled to use the Flag; the premises of diplomatic missions and consular offices, the residences of their Heads and, where appropriate, their means of official transportation.
The Laws sanctioned and promulgated by His Majesty the King, as well as the instruments signed in relation to international Treaties.
Plaques on the facades of the premises of diplomatic missions, consular offices and other diplomatic missions abroad.
The dry and wax seals of the Foreign Ministry, the credentials and patent letters and the credentials and full powers issued by the Minister of Foreign Affairs.
Certificates accrediting decorations.
The diplomas and seals for diplomas of Orders.
Official publications.
Documents, forms, stamps and letterheads for official use, with the exception of postage stamps.
The badges used by the authorities of the State to which it corresponds.
Public buildings and objects for official use in which, due to their representative nature, the symbols of the State must appear."
Anywhere else, perfectly fine to not use it.
en.wikipedia.org
